Variable magnification image tube

ABSTRACT

A variable magnification image intensifier tube (zoom tube) is disclosed. The zoom tube includes a photocathode for converting a photon image into an electron image. An image converter screen is provided for converting the electron image into an intensified optical image for viewing or use. A focus electrode, a zoom electrode and an anode electrode, are positioned between the photocathode and the image converter screen for accelerating and focusing the electron image upon the image converter screen. An electron entrance opening in the zoom electrode is dimensioned so that the electron image pattern incident on the image converter screen is in focus at both the high and low ends of the magnification range for essentially the same magnitude of focus potential applied to the focus electrode, whereby the tube does not require refocusing upon shifting from one end of the magnification scale to the other.

U nite States Patent Edgecumbe Apr. 2, 1974 VARHABLE'MAGNIFICATION IMAGETUBE Primary Examiner-J-l'erman Karl Saalbach 7s lnventor: JohnEdgecumbe, Upland, Calif. Atwmey, A88, Cole;

Pressman; Robert K. Stoddard [73] Assignee: Varian Associates, PaloAlto, Calif. I22] Filed: July 30, 1969 [57] ABSTRACT [2]] Appl. No.2846,107 A variable magnification image intensifier tube (zoom tube) isdisclosed. The zoom tube includes a photocathode for converting a photonimage into an elec- [52] US. Cl 313/102, 313/94, 313/65 R, tron image Animage converter Screen is provided for 313/82 315/15 converting theelectron image into an intensified opti- [51]" Int. Cl H103 29/46, H10 29/00 7 cal image for viewingflor useiA focus electrode. [58] Field ofSearch 313/102, 65 R, 94, 101 Zoom electrode and an anode electmde, areposi tioned between the photocathode and the image con- [56] ReferencesC'ted verter screen for accelerating and focusing the elec- UNITEDSTATES PATENT tron image upon the image converter screen. An elec-2,946,895 7/1960 Stoudenheimer et al 313/101 x tron entrance opening inthe zoom electrode is dimen- 3,082,342 3/1963 Pietri 313/94 X sioned sothat the electron image pattern incident on 3,441,786 4/1969 y /65 R Xthe image converter screen is in focus at both the high NCIlSOII X andlow ends of the magnification range for essentially gpg i i the samemagnitude of focus potential applied to the c agen e a 3,474,275 10/1969Stoudenheimer et al 313/65 ."l the tube does r r l re ocusmg uponsh1ft1ng from one end of the magnification scale to the other.

5 Claims, 6 Drawing Figures 0\/. Vi I 151w 0 12.5 KV.

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26 5 6 ANODE lMAGE IJECT RQQI E CONVERTER SC IROE EN PHOTO 25 CATlODE 19PMENFEBAPR 2 I974 3.801., 8 19 H. a W.

F +5 TO-IOOV.

IMAGE CONVERTER SCREEN :0

CATHODE lg 24 6 2005 FIGZ VPN PRIOR ART m F 3 IOO- 500- O 3 z m 200--m0- MAGNIFICATION 8 m ZOOM ELECTRODE .3 {of gF flgi g MAGNIFICATION .5|.0 400 MAGNIFICATIONK 5 Z00- Q Z BE 5 MAGNIFICATIQLN .5 L0 [ld/ m,FOCUS ZOOM ELECTRODE ELECTRODE FE INVENTOR.

w i JOHN EDGECUMBE f; j BY WM fwm 7 ATTORNEY VARIABLE MAGNIFICATIONIMAGE TUBE DESCRIPTION OF THE PRIOR ART Heretofore, variablemagnification image intensifier tubes (zoom tubes) have been constructedwith a focus electrode, a zoom electrode and an anode electrode,disposed intermediate the photocathode and the image converter screen.The magnification of the converted image in such zoom tubes has beenvariable from a low range to a high range by varying the potentialapplied to the zoom electrode. However, one of the problems with thisprior art zoom tube has been that the focus voltage required to focusthe electron image on the image converter screen was substantiallydifferent at the low end of the magnification range as opposed to thatat the high end of the magnification range. Thus, when zooming the priorart zoom tube from one end of the magnification range to the other, thetube required refocusing by adjustment of the voltage applied to thefocus electrode. It would be desirable to provide a zoom tube which maybe zoomed from one end of the magnification range to the other withoutthe requirement for refocusing.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of a variable magnification image intensifiertube having improved electron optics.

One feature of the present invention is the provision, in a variablemagnification image intensifier tube having a focus electrode, a zoomelectrode and an anode, of dimensioning the electron entrance opening inthe zoom electrode so that the electron image pattern incident on theimage converter screen is in focus at both the high and the low ends ofthe magnification range for essentially the same magnitude of focuspotential applied to the focus electrode, whereby the tube does notrequire refocusing upon zooming from one end of the magnification rangeto the other.

Another feature of the present invention is the same as the precedingfeature wherein the transverse dimensions of the electron entranceopening in the zoom electrode are smaller than those dimensions for suchan opening which would yield a lower focus potential at the high end ofthe magnification range than at the low end of the magnification range.

Another feature of the present invention is the same as the firstfeature wherein transverse dimensions of the electron entrance openingin the zoom electrode are larger than those dimensions for such anopening which would yield a higher focus potential at the high end ofthe magnification range than at the low end of the magnification range.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the focus electrode is connected tothe photocathode and operated at a constant potential, whereby thecomplexity of the zoom tube is substantially reduced.

Other features and advantages of the present invention become apparentupon a perusal of the following specification taken in connection withthe accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal sectional viewof a variable magnification image intensifier tube incorporatingfeatures of the present invention,

FIG. 2 is a plot of focus potential versus magnification depicting atypical focus voltage characteristic for a prior art image intensifiertube,

FIG. 3 is a plot similar to that of FIG. 2 depicting the focus voltagecharacteristic of an image intensifier tube incorporating the improvedelectron optics of the present invention,

FIG. 4 is a plot of focus potential versus magnification for a prior artvariable magnification image intensifier tube having optics dimensionedsuch that the required focus potential increased from the lowmagnification end of the range to the high magnification end of therange,

FIG. 5 is a plot similar to that of FIG. 4 depicting the improved focuscharacteristics obtained by the electron optics of the presentinvention, and

FIG. 6 is an alternative embodiment of a portion of the imageintensifier tube of FIG. 1 delineated by line 66 and incorporatingfeatures of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a variable magnification image intensifier tube 1 incorporatingfeatures of the present invention. The tube 1 includes a vacuum envelopestructure 2 having a faceplate structure 3 closing off one end and animage converter screen 4 closing off the other end of the vacuumenvelope 2.

The faceplate 3 includes a circular disc member made of a material whichis transparent to photons of energy emanating from, scattered from, orreflected from an object 5 to be observed. Faceplate 3 may comprise asolid disc of material or may comprise a bundle of light pipes, such asglass fibers, for observing objects out of a line-of-sight path betweenthe face of the image intensifier tube and the object. As used herein,photons are defined to include not only those photons within the visiblespectrum but those within the invisible, such as in the infrared rangeand within the higher energy range such as x-rays, gamma rays, etc. Theinside surface of the faceplate 3 is provided with a concave surfaceover which is deposited a transparent conductive film onto which isdeposited a photocathode layer 6 which is responsive to the wavelengthof the photon energy emanating from the object 5. One suitable photocathode material includes a multi alkaline photocathode material, suchas a composition of sodium, potassium, cesium and antimony. Thefaceplate disc 3, as of glass, is sealed to a metallic frame 7 as ofKovar which in turn is sealed to a sealing ring 8, as of Kovar, disposedat one end of the envelope 2.

The image converter screen 4 includes an optically transparent disc 9,as of glass or a bundle of glass fibers, having a conventionalfluorescent phosphor material 10 which fluoresces upon being impacted byelectrons deposited on an inside concave surface thereof and over whichis deposited a thin conductive layer, as of aluminum. A typical phosphorwould comprise, for example P-20 phosphor of zinc and cadmium sulfideparticles. The transparent disc 9 is sealed to a metallic frame member11, as of Kovar, which in turn is sealed, as by welding, to a Kovarframe member 12 provided at the end of the vacuum envelope structure 2.

A composite electrode structure is provided within the tube intermediatethe photocathode 6 and the image converter screen 10 for focusing andaccelerating the electron image emitted from the photocathode 6 onto theimage converter screen 10 to produce an intensified optical image at theconverter screen 10. The electrode structure includes an annular focusand gate electrode 13, followed by an annular zoom electrode 14, whichin turn is followed by an annular anode electrode 15 conductivelyconnected to frame member 12 for operation at the same potential as theimage converter screen 10.

The focus and gate electrode 13, as of stainless steel, includes anelectron entrance opening 16, at the end thereof facing the photocathode6, and a constricted electron exit opening 17. The focus and gateelectrode 13 is supported from the envelope 2 via the intermediary of anannular metallic frame member 18, as of Kovar, which forms a portion ofthe envelope 2 and a terminal for application of a focus potential, asof +300 to l volts relative to the photocathode 6, thereto. Frame 18 isinsulated from the photocathode 6 via the intermediary of a cylindricalinsulator section 19 of the envelope 2, as of glass.

The zoom electrode 14, as of stainless steel, has a constricted electronentrance opening 21 of a diameter d and which is disposed substantiallyin the plane of the constricted electron exit opening 17 in the focusand gate electrode 13. The downstream end of the zoom electrode 14includes an electron exit opening 22 which is of constricted diametercompared to the mid portion of the zoom electrode. The zoom electrode 14is carried from the vacuum envelope via an annular metallic frame 23, asof Kovar. The frame 23 is sealed into the vacuum envelope 2 via annularglass insulators 24 and 25, respectively, to permit an independentpotential as of +1.5 KV to +12.5 KV to be applied to the zoom electrode14 relative to the photocathode 6.

The anode electrode 15, as of stainless steel, includes an electronentrance opening 26 of constricted inside diameter compared to thediameter of the remaining portion of anode 15. The electron entranceopening 26 of the anode 15 is positioned within the electron exitopening 22 of the zoom electrode 14.

In operation, electron images emitted from the photocathode 6 arefocused by the focus electrode 13 through the entrance opening 21 in thezoom electrode 14 and thence focused through the electron entranceopening 26 of the anode onto the image converter screen 10. Suchelectron images are accelerated from zero potential to approximately 15KV and strike the image converter screen with a relatively high energyto produce a substantial intensification of the converted optical imageproduced at the image converter screen 10 compared to the brightness ofthe image received by the photocathode 6.

The tube 1 is operable over a certain magnification range, as of from0.3 to 1.0, in variable accordance with the potential applied to thezoom electrode 14 relative to the potential applied to the photocathode6. More particularly, with a relatively low potential applied to thezoom electrode 14, as of +1.5 KV, the entire electron image obtainedfrom the photocahtode 6 is focused over the entire surface of the imageconverter screen 10, whereas when a relatively high potential,approaching the anode potential, as of +12.5 KV, is applied to the zoomelectrode 14, only approximately a central one-third of the electronimage produced by the photocathode 6 fills the image converter screen10.

It has been discovered that there is an optimum diameter d for theelectron entrance opening 21 in the zoom electrode 14 which will permitvarying or zooming the magnification from the low end of themagnification range, as of from 0.3, to the high end of themagnification range, as of 1.0, while allowing the same focus potentialV, to be applied to the focus electrode 13 at both ends of the range.More particularly, in the prior art, the diameter d of the electronentrance opening 21 in the zoom electrode 14 was other than an optimumdiameter such that the focus potential had to be changed for eachdifferent setting of the magnification and was different at both ends ofthe range. For example, when the diameter d of the electron entranceopening 21 was larger than the optimum diameter, the focus potentialcharacteristic was as shown in FIG. 2, whereas when the diameter of theelectron entrance opening 21 was smaller than the optimum diameter thefocus potential characteristic was as shown in FIG. 4.

This prior art focus potential versus magnification characteristic wastroublesome since when zooming the tube from one end of the range to theother the focus potential had to be changed to obtain a focus of theimage. However, when the diameter d of the electron entrance opening 21in the zoom electrode 14 is selected, during fabrication of the tube, tohave the optimum diameter, the focus potential is the same at the lowend of the magnification range as it is at the high end of themagnification range, as shown in FIGS. 3 and 5. For the focuscharacteristic as shown in FIG. 2, the optimum diameter for the electronentrance opening 21 in the zoom electrode 14 is decreased to obtain thecharacteristic for the optimum diameter, as shown in FIG. 3. When thefocus potential characteristic is as shown in FIG. 4 the diameter 3 forthe electron entrance opening 21 in the zoom electrode 14 is increasedto obtain the characteristic as shown in FIG. 5. In a typical example,wherein the photocathode 6 had a diameter of 6 inches, the prior artelectron entrance opening 21 of the zoom electrode 14 had a diameter of1.65 inches. This resulted in a focus potential characteristic as shownin FIG. 2. By decreasing the diameter d of the electron entrance opening21 of the zoom electrode from 1.65 to 1.00 inch the-focus characteristicwas changed to that as shown in FIG. 3 such that the focus potential wasthe same at the low and high ends of the magnification range, therebyavoiding the necessity of refocusing tube on zooming from the low to thehigh end of the magnification range.

In the tube of FIG. 1 the structure is essentially a tetrode in thatfour independent potentials are applied to the electrode structures. Thefocus electrode 13 is also employed as a gate for gating on and off theelectron image to the image converter screen 10 for controlling the gainof the image intensifier tube. The discovery of an optimum diameter forthe electron entrance opening 21 in the zoom electrode 14 permitsconstruction of a triode image intensifier tube, provided that gaincontrol is not desirable. Such a triode construction is depicted in FIG.6.

Referring now to FIG. 6, there is shown an alternative triode embodimentof the present invention wherein the structure is essentially the sameas that of FIG. 1 except that the focus electrode 13 is conductivelyconnected to the photocathode 6 and operated at photocathode potential.The focus electrode 13 is dimensioned relative to the dimensions of thezoom electrode 14 and to the dimensions of the photocathode 6 such thatat one end of the magnification range, the focus potential is volts. Thediameter d of the electron entrance opening 21 of the zoom electrode 14is then dimensioned for the optimum diameter such that the 0 volt focuspotential is the same at the high and low ends of the magnificationrange. In this manner a separate focus potential is not required and theconstruction of the tube is substantially simplified, In thisembodiment, the annular frame member 8 is extended to the frame member18 which in turn holds the focus electrode 13.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a variable magnification image intensifier tube, means forming aphotocathode for converting a photon input image incident upon saidphotocathode into an electron image pattern emitted from saidphotocathode, an image converter screen for converting the electronimage pattern into an intensified output optical image, means forming anelectrode structure disposed intermediate said photocathode and saidimage converter screen for accelerating and focusing the emittedelectron image pattern onto said image converter screen, said electrodestructure including an anode electrode means, a zoom electrode means,and a focus electrode means, said zoom electrode means being disposedbetween said photocathode and said anode electrode for varying themagnification of the electron image incident on said image converterscreen over a certain range of magnification having a high end and a lowend in variable accordance with a potential applied to said zoomelectrode means, said zoom electrode having an electron entrance openingand an electron exit opening through which electron images pass to saidconverter screen, said focus electrode means being disposed imtermediatesaid zoom electrode and said photocathode for focusing the electronimage upon said image converter screen,

the improvement WHEREIN, the size of said electron entrance opening isdimensioned such that the electron image pattern incident on said imageconverter screen is in focus at both the high and low ends of saidmagnification range for substantially the same magnitude of focuspotential applied to said focus electrode means.

2. The apparatus of claim 1 wherein said photocathode is concave havinga concave surface facing said zoom electrode means, said focus electrodeis annular having an electron entrance opening and an electron exitopening of smaller dimensions than said electron entrance opening insaid focus electrode means, and wherein said electron entrance openingin said zoom electrode is positioned substantially at said electron exitopening of said focus electrode means and is of constricted transversedimensions compared to the transverse dimensions of the midportion ofthe electron passageway through said zoom electrode means.

3. The apparatus of claim 2 wherein said anode electrode means isannular having an electron entrance opening and an electron exitopening, said electron entrance opening in said anode e lec t ro debeingof smaller trah sfiafse (ii r;e ions than said electronTit opening ofsaid anode electrode, and said electron entrance opening of said anodeelectrode being disposed within the electron passageway of said zoomelectrode means.

4. The apparatus of claim 1 wherein said transverse dimensions of saidelectron entrance opening in said zoom electrode are smaller than thosedimensions for such an opening which would yield a lower focus potentialat the high end of the magnification range than at the low end of themagnification range.

5. The apparatus of claim 1 wherein said transverse dimensions of saidelectron entrance opening in said zoom electrode are larger than thosedimensions for such an opening which would yield a higher focus po'tential at the high end of the magnification range than at the low endof the magnification range.

1. In a variable magnification image intensifier tube, means forming aphotocathode for converting a photon input image incident upon saidphotocathode into an electron image pattern emitted from saidphotocathode, an image converter screen for converting the electronimage pattern into an intensified output optical image, means forming anelectrode structure disposed intermediate said photocathode and saidimage converter screen for accelerating and focusing the emittedelectron image pattern onto said image converter screen, said electrodestructure including an anode electrode means, a zoom electrode means,and a focus electrode means, said zoom electrode means being disposedbetween said photocathode and said anode electrode for varying themagnification of the electron image incident on said image converterscreen over a certain range of magnification having a high end and a lowend in variable accordance with a potential applied to said zoomelectrode means, said zoom electrode having an electron entrance openingand an electron exit opening through which electron images pass to saidconverter screen, said focus electrode means being disposed imtermediatesaid zoom electrode and said photocathode for focusing the electronimage upon said image converter screen, the improvement WHEREIN, thesize of said electron entrance opening is dimensioned such that theelectron image pattern incident on said image converter screen is infocus at both the high and low ends of said magnification range forsubstantially the same magnitude of focus potential applied to saidfocus electrode means.
 2. The apparatus of claim 1 wherein saidphotocathode is concave having a concave surface facing said zoomelectrode means, said focus electrode is annular having an electronentrance opening and an electron exit opening of smaller dimensions thansaid electron entrance opening in said focus electrode means, andwherein said electron entrance opening in said zoom electrode ispositioned substantially at said electron exit opening of said focuselectrode means and is of constricted transverse dimensions compared tothe transverse dimensions of the midportion of the electron passagewaythrough said zoom electrode means.
 3. The apparatus of claim 2 whereinsaid anode electrode means is annular having an electron entranceopening and an electron exit opening, said electron entrance opening insaid anode electrode being of samller transverse dimensions than saidelectron exit opening of said anode electrode, and said electronentrance opening of said anode electrode being disposed within theelectron passageway of said zoom electrode means.
 4. The apparatus ofclaim 1 wherein said transverse dimensions of said electron entranceopening in said zoom electrode are smaller than those dimensions forsuch an opening which would yield a lower focus potential at the highend of the magnification range than at the low end of the magnificationrange.
 5. The apparatus of claim 1 wherein said transverse dimensions ofsaid electron entrance opening in said zoom electrode are larger thanthose dimensions for such an opening which would yield a higher focuspotential at the high end of the magnification range than at the low endof the magnification range.